Psittacosaurus
Psittacosaurus (pronounced /ˌsɪtəkɵˈsɔrəs/, from the Greek for 'parrot lizard') is a genus of psittacosaurid ceratopsian dinosaur from the Early Cretaceous Period of what is now Asia, about 130 to 100 million years ago. It is notable for being the most species-rich dinosaur genus. At least ten extinct species are recognized from fossils found in different regions of modern-day China, Mongolia and Russia, with a possible additional species from Thailand. All species of Psittacosaurus were gazelle-sized bipedal herbivores characterized by a high, powerful beak on the upper jaw. At least one species had long, quill-like structures on its tail and lower back, possibly serving a display function. Psittacosaurs were extremely early ceratopsians and, while they developed many novel adaptations of their own, they also shared many anatomical features with later ceratopsians, such as Protoceratops and the elephant-sized Triceratops. Psittacosaurus is not as familiar to the general public as its distant relative Triceratops but it is one of the most completely known dinosaur genera. Fossils of over 400 individuals have been collected so far, including many complete skeletons. Most different age classes are represented, from hatchling through to adult, which has allowed several detailed studies of Psittacosaurus growth rates and reproductive biology. The abundance of this dinosaur in the fossil record has led to its use as an index fossil for Early Cretaceous sediments of central Asia. Description Different species of Psittacosaurus varied in size and specific features of the skull and skeleton, but shared the same overall body shape. The best-known species, P. mongoliensis, reached 2 meters (6.5 ft) in length. The maximum adult body weight was most likely over 20 kilograms (44 lb) in P. mongoliensis. Several species approached P. mongoliensis in size (P. major, P. neimongoliensis, P. xinjiangensis),456 while others were somewhat smaller (P. sinensis, P. meileyingensis). P. ordosensis was the smallest known species, 30% smaller than P. mongoliensis. The largest were P. lujiatunensis and P. sibiricus, although neither was significantly larger than P. mongoliensis. The skull of Psittacosaurus was highly modified compared to other ornithischian dinosaurs of its time. The skull was extremely tall and short, with an almost round profile in some species. The portion in front of the orbit (eye socket) was only 40% of total skull length, shorter than any other known ornithischian. The lower jaws of psittacosaurs are characterized by a bulbous vertical ridge down the center of each tooth. Both upper and lower jaws sported a pronounced beak, formed from the rostral and predentary bones, respectively. The bony core of the beak may have been sheathed in keratin to provide a sharp cutting surface for cropping plant material. As the generic name suggests, the short skull and beak superficially resembled those of modern parrots. Psittacosaurus skulls shared several adaptations with more derived ceratopsians, such as the unique rostral bone at the tip of the upper jaw, and the flared jugal (cheek) bones. However, there was still no sign of the bony neck frill or prominent facial horns which would develop in later ceratopsians.10 Bony horns did protrude from the skull of P. sibiricus, but these are thought to be an example of convergent evolution. Psittacosaurus postcranial skeletons were more typical of a 'generic' bipedal ornithischian. In P. mongoliensis, similarly to other species, the forelimbs were only 58% as long as the hindlimbs, indicating that these animals were almost totally bipedal in life. There were only four digits on the manus ('hand'), as opposed to the five found in most other ornithischians (including all other ceratopsians). Overall, the four-toed hindfoot was very similar to many other small ornithischians. Taxonomy Psittacosaurus was named in 1923 by Henry Fairfield Osborn, paleontologist and president of the American Museum of Natural History (AMNH) in a paper published on October 19. The generic name is composed of the Greek words ψιττακος/psittakos ('parrot') and σαυρος/sauros ('lizard'), suggested by the superficially parrot-like beak of these animals and their reptilian nature. Species of Psittacosaurus Main article: Species of Psittacosaurus Life reconstructions of eight species of Psittacosaurus, drawn to scale.Over a dozen species have been referred to the genus Psittacosaurus, although only nine to eleven are considered valid today. This is the highest number of valid species currently assigned to any single dinosaur genus (not including birds). In contrast, most other dinosaur genera are monospecific, containing only a single known species. The difference is most likely due to quirks of the fossil record. While Psittacosaurus is known from hundreds of fossil specimens, most other dinosaur species are known from far fewer, and many are represented by only a single specimen. With a very high sample size, the diversity of Psittacosaurus can be analyzed more completely than that of most dinosaur genera, resulting in the recognition of more species. Most extant animal genera are represented by multiple species, suggesting that this may have been the case for extinct dinosaur genera as well, although most of these species may not have been preserved. In addition, most dinosaurs are known solely from bones and can only be evaluated from a morphological standpoint, whereas extant species often have very similar skeletal morphology but differ in other ways which would not normally be preserved in the fossil record, such as behavior, or coloration. Therefore actual species diversity may be much higher than currently recognized in this and other dinosaur genera. * Valid Psittacosaurus species * Psittacosaurus mongoliensis — Mongolia, northern China * Psittacosaurus sinensis — northeastern China *Psittacosaurus meileyingensis — north-central China * Psittacosaurus xinjiangensis — northwestern China * Psittacosaurus neimongoliensis — north-central China * Psittacosaurus ordosensis — north-central China * Psittacosaurus mazongshanensis — northwestern China * Psittacosaurus sibiricus - Russia (southern Siberia) * Psittacosaurus lujiatunensis - northeastern China * Psittacosaurus major - northeastern China * Psittacosaurus gobiensis - Inner Mongolia * Possible Psittacosaurus species *?Psittacosaurus sattayaraki - Thailand Classification Psittacosaurus is the type genus of the family Psittacosauridae, which was also named by Osborn in 1923. Only one other genus, Hongshanosaurus, is currently classified in this family alongside Psittacosaurus.22 Psittacosaurids were basal to almost all known ceratopsians except Yinlong and perhaps Chaoyangsauridae. While Psittacosauridae was an early branch of the ceratopsian family tree, Psittacosaurus itself was probably not directly ancestral to any other groups of ceratopsians. All other ceratopsians retained the fifth digit of the hand, a plesiomorphy or primitive trait, whereas all species of Psittacosaurus had only four digits on the hand. In addition, the antorbital fenestra, an opening in the skull between the eye socket and nostril, was lost during the evolution of Psittacosauridae, but is still found in most other ceratopsians and in fact most other archosaurs. It is considered highly unlikely that the fifth digit or antorbital fenestra would evolve a second time. Although many species of Psittacosaurus have been named, their relationships to each other have not yet been fully explored and no scientific consensus exists on the subject. The most recent and most detailed cladistic analysis was published by Alexander Averianov and colleagues in 2006: It has been suggested that P. lujiatunensis is basal to all other species. This would be consistent with its earlier appearance in the fossil record.[